1. Technical Field of the Invention
The present invention relates to a speakerphone which enables a user to talk to the other party over the telephone without lifting a handset and to a microphone case for use in the same.
2. Prior Art
Heretofore, there has been known a speakerphone which incorporates a speaker and a microphone for amplifying a voice in a housing to enable a user to talk to the other party over the telephone without using a handset.
In general, in a speakerphone, a speaker and a microphone are incorporated at a relatively close distance in a same housing. Therefore, there is available a speakerphone which incorporates a voice switch circuit to prevent acoustic coupling between them, that is, a phenomenon that the voice of the other party output from the speaker is picked up by the microphone. FIG. 17 is a block diagram showing a configuration of a speakerphone incorporating a voice switch circuit. As shown in this figure, the speakerphone comprises a hybrid circuit 1, a voice switch circuit 2, a speaker amplifier for amplifying a voice (to be referred to as "speaker amplifier" hereinafter) 3, a louder speaker for outputting an amplified voice (to be referred to as "speaker" hereinafter) 4, a microphone amplifier for amplifying a voice (to be referred to as "microphone amplifier" hereinafter) 5, and a speech microphone for inputting a voice to be amplified (to be referred to as "microphone" hereinafter) 6.
The hybrid circuit 1 is an interface for connecting the speakerphone to a telephone line network 10, which outputs a received signal to the voice switch circuit 2 and a transmitted signal to the telephone line network 10 and prevents the transmitted signal from being input into the voice switch circuit 2. The voice switch circuit 2 reduces the transmitted signal level during reception and the received signal level during transmission, and comprises a reception suppression circuit 11, a transmission suppression circuit 12 and a control circuit 13. The control circuit 13 compares a transmitted signal and a received signal, controls the reception suppression circuit 11 to attenuate the received signal during transmission (transmitted signal level&gt;received signal level) and controls the transmission suppression circuit 12 to attenuate the transmitted signal during reception (received signal level&gt;transmitted signal level).
Since an audio output from the microphone 6 can be suppressed by the audio switch circuit 2 even when a voice from the speaker 4 is picked up by the microphone 6, acoustic coupling between the speaker 4 and the microphone 6 can be suppressed. FIG. 18 shows a block diagram illustrating acoustic coupling between the speaker 4 and the microphone 6. As shown in the figure, a received signal Sr from the other party is output from the speaker 4 on a user's side as an audio signal, the audio signal is picked up by the microphone 6 on the user's side and output to the other party as a transmitted signal St. In this case, without any voice switch circuit 2, the level of the transmitted signal St input from the microphone 6 may become larger by picking up the received signal, resulting in the occurrence of howling or acoustic echo. However, as this conventional speakerphone incorporates the voice switch circuit 2, the level of the transmitted signal St picked up by the microphone 6 is reduced, thereby suppressing the occurrence of howling or acoustic echo.
As shown in FIG. 17, the speaker amplifier 3 amplifies a received signal from the voice switch circuit 2 to a predetermined level, while the microphone amplifier 5 amplifies a transmitted signal from the microphone 6 to a predetermined level. In this case, when an appropriate speech acoustic level is 74 dBSPL (SPL; sound pressure level), a gain for the speaker amplifier 3 is set to produce an acoustic output from the speaker 4 larger than 94 dB so that a received voice from the speaker 4 is heard at 74 dBSPL. Furthermore, since an acoustic level of ca. 94 dB is added to the microphone 6 when a user speaks, a gain for the microphone amplifier 5 is set such that the acoustic level should become the regulated audio level of a telephone circuit. The received signal amplified by the speaker amplifier 3 is supplied to the speaker 4 to generate a sound while the transmitted signal amplified by the microphone amplifier 5 is supplied to the voice switch circuit 2.
FIG. 19 is a perspective view of the outer appearance of the speakerphone. As shown in this figure, sound output holes 16 for the speaker are formed in a center portion on a front left side of the housing 15, and a sound input hole 17 for the microphone is formed in a right portion on a front side of the housing 15. The speaker 4 is arranged inside the housing 15 at a position where the sound output holes are formed and the microphone 6 is arranged inside the housing 15 at a position where the sound input hole 17 is formed.
FIG. 20 is a transverse sectional view of FIG. 19, FIG. 21 is a sectional view of the key parts of FIG. 19, and FIG. 22 is an enlarged sectional view of a portion where the microphone 6 is installed. In these figures, the microphone 6 is stored in a rubber case 20, and a sound introduction surface 6A (see FIG. 22) of the microphone 6 is arranged to face the sound input hole 17. A wire 6B of the microphone 6 is connected to a circuit board 22. The rubber case 20 excludes sounds output into the inside of the housing 15 and covers all the portions of the microphone 6 except the sound introduction surface 6A. Further, a handset 18 is generally provided to the speakerphone in addition to the speaker 4 and the microphone 6 for amplifying a voice so that the user can talk through the handset 18.
However, the above conventional speakerphone has the following problems. When the user talks over the telephone at an appropriate speech level, most of a voice from the speaker 4 may be picked up by the microphone 6 and the other party may hear his/her voice like an acoustic echo. According to fluctuations in the AC impedance of a telephone circuit, a transmitted signal may leak into a received signal to generate a hybrid echo in the hybrid circuit 1. Therefore, the sound suppression loss of the voice switch circuit 2 is set large in the prior art. However, by setting the sound suppression loss large, a slight lag is produced when priority is shifted from the speaker 4 to the microphone 6, and a first portion of a word input from the microphone 6 is missing, whereby speech becomes unnatural.
Further, when big sounds are output from the speaker 4, sounds output from the rear side of the speaker 4 may stay in the inside of the housing 15 of the speakerphone and form an effective acoustic coupling passage with the microphone 6. However, it is extremely difficult to cancel it. Even when the rear side of the microphone 6 is covered by the rubber case 20, sounds from the speaker 4 are transmitted too large to the microphone 6 through a cover and base forming the housing 15 and mechanical parts such as a circuit board. If mechanical vibrations of the cover, base, printing circuit board and the like are transmitted too large to the microphone 6, the voice switch circuit 2 carries out transmission operation, the switch is electrically kept switched to the microphone 6 in the worst case, and normal communication becomes impossible.
To solve this problem, the following methods have been considered: (1) the distance between the speaker 4 and the microphone 6 is increased, (2) the rear of the speaker 4 is completely covered by another part, (3) the microphone 6 is removed from the housing 15 and installed outside the housing 15 to eliminate acoustic coupling within the housing 15, and (4) a special directional microphone is used.
However, in either case, there is no design freedom because the housing 15 becomes bulky, the number of parts is larger than required by using expensive parts, and there is no installation space inside the housing 15. Therefore, there is no definite solution to the above problems.
A speakerphone which uses an acoustic echo chancellor to reduce the voice suppression loss of the voice switch circuit 2 is conceivable. The acoustic echo chancellor cannot obtain sufficient performance (the performance of preventing sounds from the speaker 4 from being picked up by the microphone 6) if the acoustic coupling volume is large. When an acoustic echo chancellor is used, the speaker 4 and the microphone 6 must be separated from each other, and it is difficult to incorporate them in the telephone.
The present invention has been made to solve the problems of the above speakerphone and it is an object of the present invention to provide a speakerphone which can obtain appropriate speech characteristics against acoustic coupling between the speaker and the microphone. It is another object of the present invention to provide a microphone case which is effective in improving the above speech characteristics.